This invention relates to the art of web fed printing presses and, more particularly, to motor control circuitry for controlling the speed of web material as it is passed through a press unit and is then subsequently redirected, as to a utilization device, such as a folder.
Whereas the invention is described herein with particular reference to controlling the speed relationship of a motor driven press line shaft and a motor driven nip roll, it is to be appreciated that the invention may be used in various press applications requiring the ability to vary the web speed relationship at different locations in the path of movement of a web.
In a web fed printing press it is typical for a web to be fed through one or more printing units where impressions are made in one or more colored inks with the web then being directed to a nip roll prior to being supplied to a utilization device, such as a folder. The nip rolls (frequently referred to as pinch rolls where one is driven and the other is an idler) may be set to impart, by a pulling force, a slightly greater surface speed to the web then that imparted by the press unit. This is done to maintain proper tension. Thus, as the web is fed through the press unit it is dampened by the wetting effect taking place during the printing operations and this will cause the web to stretch slightly. In order to maintain proper tension, the nip rolls impart a slightly increased surface speed.
Conventionally, the nip rolls are geared to the press line shaft which is used to drive all of the press units. In order to achieve a slightly different web speed at the nip rolls, a variation may be made in the gearing ratio interconnecting the press line shaft with the nip rolls. However, it is more conventional to employ nip rolls of a diameter to achieve the web surface speed desired. This, however, does not permit easy adjustment in relative speed between nip rolls and the press unit as any such adjustment will require changing the nip rolls or changing the gearing between the nip rolls and the press line shaft. This is awkward and time consuming. If the nip rolls be motor driven independently of the motor driven press line shaft, then the motors must be synchronized. Motor control systems for synchronizing the speed of motors are well-known and it is conventional to use a phase lock loop technique. This requires that a pulse encoder be connected to the motor shaft at each motor so that the motor speeds may be synchronized or locked by supplying pulses from the encoders to a phase lock loop circuit. Here, the two pulse trains are compared and if the relationship is not correct, a corrective signal is supplied to one of the motors to speed it up or slow it down until pulse synchronization is obtained.
Such a phase lock loop motor control system will not, however, permit easy adjustment of the relative speed of the nip rolls with respect to the press unit in the press environment discussed above. To achieve relative speed adjustment will require some way of varying the number of pulses per revolution produced by one of the shaft encoders. This then would then require changing the shaft encoders in order to achieve different relative speeds.
It is therefore a primary object of the present invention to provide adjustments in the relative speed of a web at different locations without making changes in gear ratios or nip roll diameter.
It is a further object to employ a motor driven press line shaft and a motor driven nip roll and to adjust the speed relationship therebetween without changing one of the motor driven encoders.
The present invention contemplates the provision of a web fed printing press having at least one printing unit. A first motor drives, as by way of a press line shaft, a first web driving means, such as a printing cylinder-blanket cylinder couple in the press unit, so as to impart motion to the web. Downstream from the printing unit, a second web driving means, such as nip rolls, driven by a second motor also imparts motion to the web prior to it being directed to a utilization device, such as a folder.
In accordance with the present invention, the relative web speed at the two motor driven means is controlled. A first pulse train is provided at a pulse rate dependent on the speed of the first motor and a second pulse train is provided dependent on the speed of the second motor. These pulse trains are compared for motor speed synchronism and if the speed relationship is not correct then the energization of the second motor is varied to achieve the proper speed relationship. The relative speed may be varied by effectively changing the number of pulses per revolution in the second pulse train.